Acoustic well logging is a generally accepted method for obtaining information about subterranean earth formations surrounding wells or boreholes. Acoustic well logging can be used in the determination of formation lithology, density, porosity, the conversion of seismic time sections to depth sections, and the detection of fractures.
A conventional acoustic logging system includes a logging sonde suspended in the borehole liquid, a source attached to the sonde for generating compressional waves ("P-waves"), and two or more receivers attached to the sonde and spaced apart from the P-wave source for detecting P-waves in the borehole liquid. A P-wave in the borehole liquid generated by the source is refracted in the earth formation surrounding the borehole. The refracted wave propagates through a portion of the formation, is refracted back into the borehole liquid, and detected by two or more receivers spaced vertically apart from each other and from the P-wave source. The first arrival will be the P-wave which refracts along the borehole wall. The ratio of the separation between the two receivers to the time difference between the detections of the refracted P-wave by the two receivers yields the P-wave velocity in the formation. From this information, many characteristics of the formation can be determined.
Conventional acoustic logging is dependent on the respective velocities of the P-waves in the borehole liquid ("liquid velocity") and in the surrounding earth formation ("formation velocity"). Refraction necessary to facilitate acoustic well logging will not occur if the liquid velocity exceeds the formation velocity. The wave will not appropriately refract in the formation so that signals can be detected. Formation velocity is often less than liquid velocity in the top levels of a formation. For this reason, conventional acoustic logging cannot be performed in many formations in the first few hundred feet below the surface.
While acoustic well logging is a widespread method of collecting data, no conventional methods permit acoustic logging in formations where liquid velocity exceeds formation velocity ("low velocity formation"). All conventional acoustic logging methods require a formation velocity that is higher than the liquid velocity. The present invention is a method and apparatus for solving this problem.